1. Field of the Invention
The present invention generally relates to a light emitting diode and manufacturing method thereof. More particularly, the present invention relates to a substrate-free flip chip light emitting diode and manufacturing method thereof.
2. Description of Related Art
Light emitting diode (LED) is a semiconductor component that has been broadly used as light emission device. The light emitting chip of the LED is generally comprised of III-V compound semiconductor such as gallium phosphide (GaP), gallium arsenide (GaAs), or gallium nitride (GaN). The principle of light emission of LED is the transformation of electrical energy into light energy, which is performed by applying current to the compound semiconductor to generate electrons and holes. Thereafter, an excess energy is released by the combination of electrons and holes, and thus the LED emits light. In general, the LED has the advantages of fast response speed (generally about 10−9 seconds), excellent monochromatic color light, small size, low power consumption, low contamination (mercury free), high reliability, and the manufacturing process is suitable for mass production. Therefore, the application of LED is very broad and includes, for example, traffic light, large size displaying billboard and the display of many portable electronic devices.
In principle, a fundamental structure of a LED device includes an epitaxy layer of a P-type and a N-type III-V group compound and a light emitting layer in-between. The light emitting efficiency of the LED device is dependent on the internal quantum efficiency of the light emitting layer and the light extraction efficiency of the device. A method of increasing the internal quantum efficiency includes, for the most part, improving the quality of the light emitting layer and the design of the structure. The method of increasing the light extraction efficiency includes, for the most part, decreasing the light loss caused by the absorption of the light emitted from the light emitting layer due to the reflection of the light inside the LED device.
Conventionally, a variety of LED structures and manufacturing methods has been developed. Hereinafter, an exemplary embodiment showing the LED structure and the manufacturing method thereof according to the U.S. Pat. No. 6,462,358 will be described. FIG. 1A to FIG. 1B are cross-sectional views schematically illustrating a manufacturing process of a conventional LED device. Referring to FIG. 1A, The epitaxial structure 100 includes an N-type GaAs substrate 102, an etching stop layer 104, an N-type (AlxGa1-x)0.5In0.5P lower cladding layer 106 (0.5≦x≦1.0), a (AlxGa1-x)0.5In0.5P active layer 108 (0≦x≦0.45), a P-type (AlxGa1-x)0.5In0.5P upper cladding layer 110 (0.5≦≦1.0), a P-type epitaxial layer 112 and a plurality P-type ohmic contacts 114a and 114b. 
Next, referring to FIG. 1B, a transparent adhesive layer 122 and a transparent substrate 124 are formed over the P-type epitaxial layer 112 and covers the p-type ohmic contacts 114a and 114b. The transparent substrate 124 is connected to the P-type ohmic contacts 114a and 114b and the epitaxial layer 112 by pressuring and heating the transparent adhesive layer 122 at 250° C. for a while. The transparent adhesive layer 122 is composed of B-staged bisbenzocyclobutene (BCB) or other transparent adhesive materials such as epoxy. The transparent substrate 124 includes polycrystal substrate or amorphous substrate, such as sapphire, glass, GaP, GaAsP, ZnSe, ZnS, ZnSSe, or SiC substrate.
Then, the substrate 102 is etched by a corrosive etchant. If the etching stop layer 104 is made of light-absorption materials, such as GalnP or AlGaAs, the etching stop layer 104 must be removed by the same solution. Then, referring to FIG. 1B, a portion of the lower cladding layer 106, the active layer 108 and the upper cladding layer 110 is removed by dry etching or wet etching process to expose a portion of the epitaxial layer 112. Subsequently, the lower portion of the exposed epitaxial layer 112 is removed to form a channel 132 exposing the P-type ohmic contact 114b. Then, an N-type ohmic contact 134 is formed on the lower cladding layer 106. Thereafter, a first metal bonding layer 136 is formed on the epitaxial layer 112 and the channel 132 is filled by Au or Al to form an electrode channel 132 connecting the P-type ohmic contact 114b. Then, a second metal bonding layer 138 is formed on the N-type ohmic contact layer 134. Finally, a LED epitaxial structure 150 is formed.
Accordingly, in the manufacturing process of conventional LED device, since the adhesive layer 122 and the substrate 124 have to be transparent, the heat-sink efficiency of the material of the adhesive layer 122 and the substrate 124 described above is poor. Thus, the life period of the conventional LED device is reduced. In addition, the substrate having a better light transmittance such as sapphire is very expensive, if a substance having poor light transmittance is provided, the cost may be reduced but the light emitting efficiency of the conventional LED device is also reduced. Moreover, to enhance the light transmittance of the conventional LED device, the surface of the transparent substrate 124 has to be polished, thus the process is more complex and the yield of the process is reduced.